Mobile cellular communication systems have become increasingly important, providing mobile users the security of being able to communicate quickly and reliably from almost any location. Present cellular communication systems use terrestrial transmitters, such as base stations, fixed sites or towers, to define each cell of the system, so that the extent of a particular cellular communication system is limited by the region over which the base stations are distributed. Many parts of the world are relatively inaccessible, or, as in the case of the ocean, do not lend themselves to location of a plurality of dispersed cellular sites. In these regions of the world, spacecraft-based communication systems may be preferable to terrestrial-based systems. It is desirable that a spacecraft cellular communications system adhere, insofar as possible, to the standards which are common to terrestrial systems, and in particular to such systems as the Global System for Mobile Communications (GSM), which is in use in Europe.
A cellular communication system should provide a channel for allowing a user terminal to initiate communications with the network. Each base station, fixed site, or tower continually transmits network synchronization information (SCH) and network-specific information (BCCH), which a user terminal uses to synchronize to the appropriate network at initial turn-on of the user terminal. The GSM system provides a control channel denominated “Random Access Channel” or RACH. In GSM, the RACH channel is used for initial synchronization of the network to the user terminal. To implement the RACH, the user terminal sends to the based station an “access burst,” which includes a finite duration modulated carrier, transmitted in one TDMA time slot, carrying information. In the GSM terrestrial system, the burst includes eight beginning tail bits, forty-one synchronization bits, thirty-six coded data bits, and three ending tail bits. In GSM, the slot duration is 156.25 bits long, so the access burst has 68.25 bits of guard time. The thirty-six coded data bits allow coding, using R=½, K=5 convolutional code, of a group of eight bits of information, six bits of cyclic redundancy code (CRC) for error detection, and four bits of decoder trellis termination.
Aloha type contention channel access is the preferred method of terminal access in cellular mobile systems today. This is due to reasons of simplicity, among others. In a slotted aloha system, a terminal's transmitter and receiver must be synchronized to a network timing reference. Because cellular systems are typically characterized by small cell sizes on the order of a mile radius, the timing uncertainty between a terminal and the network timing reference is limited to a few milliseconds.
Satellite communication systems, however, introduce additional factors which must be taken into account in order for the system to function properly. In a satellite communication system, the timing uncertainty of each terminal is greatly increased due to large variations in the total path delay. As shown in FIG. 1, in a geostationary satellite system the total path delay from a user terminal to the satellite varies from approximately 117 msec for terminals immediately below the satellite to approximately 135 msec at the edge of coverage. Thus, the total round trip variation, or uncertainty, in path delay is on the order of 36 msec. In other words, in a geostationary satellite system, the potential error between a terminal's transmit timing reference and the system timing can be as bad as 36 msec without closed loop correction.
Using spot beams to reduce the geographic distance between terminals transmitting on a known beam can reduce the timing uncertainty to a few milliseconds. However, the remaining timing uncertainty is still significantly greater than the uncertainty present in typical land-based systems, and satellite communication systems must account for the increased timing uncertainty.
One method used to account for the increased timing uncertainty in satellite communication systems is to increase the time window in which Random Access Channel (RACH) bursts can be received from earth based terminals at the satellite. Such a method is used in the Thuraya satellite system, which is based on the GMR-1 air interface standard, described in European Telecommunications Standard Institute Document No. TS-101 376.
FIG. 2 illustrates the frequency-time mapping of a typical frame in a GSM radio network. A 200 kHz channel is time divided into 4.62 millisecond frames. Each frame is further divided into eight (8) time slots. One time slot is reserved as a control channel to receive RACH bursts from terminals. The remaining seven (7) time slots are used as traffic channels. Thus, in the GSM system, control channel overhead accounts for 12.5% of the capacity of each 200 kHz channel. Unfortunately, the duration of the control channel in the GSM system is insufficient to account for the timing uncertainties discussed above for satellite based communication networks.
The AceS satellite system, shown in FIG. 3, operates under the GMR-2 air interface standard, described in ETSI document No. TS-101 377. The return link in the AceS system divides a 200 kHz channel into four 50 kHz subchannels. One of the subchannels is reserved for contention channel time slots. The number of time slots can be defined in a given system to have any number of time slots out of the eight time slot frame. Thus, in the AceS system, typically a four time slot contention channel is reserved in the sub channel reserved for contention access, resulting in a 2.31 msec, 50 kHz contention channel. Unfortunately, the subchannels in the AceS system are of insufficient bandwidth for a broadband data system. Furthermore, even if all eight time slots for a given sub channel were reserved as a contention channel, the 4.62 msec frame duration is still insufficient to account for the timing uncertainty among terminals spread out over the earth, which can be on the order of 6 msec, even among terminals within a certain spot beam.
The Thuraya satellite system solved the timing uncertainty problem by incorporating a large contention channel window with sufficient guard time and a contention channel burst design with sufficient synchronization pattern overhead. This solution was adequate for a circuit switched, narrow band system.
Unfortunately, this solution is insufficient for a broadband packet switched system for two reasons. First, in a broadband system, the channels are designed to have broader bandwidth to accommodate broadband traffic. Thus, the additional guard time needed to accommodate for the increased timing uncertainties associated with satellite based systems is much more “expensive” due to the broader bandwidth associated with the additional guard time. Secondly, packet switched traffic is bursty by nature, as compared to circuit switched systems. Thus, the volume of connections are dramatically increased, causing a corresponding increase in demand on the contention channel.
Accordingly, there is a need for a contention channel design in a satellite based broadband packet switched communication system which effectively compensates for the increased timing uncertainty of satellite transmissions, while avoiding unnecessary waste of bandwidth, and accommodating the increased contention channel demand of a packet switched system.